26m servers in data centres use 2% of world electricity

BT’s green credentials are well established. It is the largest commercial buyer of renewable electricity in the UK, emphasises the importance of carbon reduction across the organisation, and pushes voice and video conferencing at an unconvinced customer base. In any international ranking, BT’s sustainability measures get high marks. But BT has the same problem as many other organisations: its server farms are growing in number and size. The increased power consumption in its data centres explains why the organisation’s electricity demand is growing. Eventually, its brand image will suffer as critics suggest that its public stance on green issues is not matched by its internal behaviour.

BT’s electricity use is about half a percent of the UK’s total, and its server farms represent over 10% of its energy consumption. BT says that data centre use is rising at 40% a year, and the company’s emphasis on growing video businesses, such as BT Vision, is likely to increase data storage and transmission demands into the foreseeable future.

BT’s response has been to attack the power use of the server with radical measures that set best practice elsewhere in the world. Its new data centres use fresh air cooling, not air conditioning, and the company runs its machines at much higher temperatures than used to be considered possible. Since cooling servers uses at least as much power as running them, this is an important step. The second major innovation is to run the farms on DC power, cutting the very significant losses in the multiple AC to DC conversions in a conventional centre. Better ‘loading’ of the computers helps as well. A well-utilised machine uses only a little more power than an intermittently under-employed server. BT claims that these measures can reduce the typical power consumption of a server farm by 60%.

Across the world, data centre energy consumption is becoming a bigger issue. The world has about 26m servers pumping out data day and night. Estimates suggest that they use about 2% of all electricity produced and global growth is probably around 15% a year. BT’s innovations may be a useful model for others to follow. But the unfortunate fact is that at current growth rates the maximum efficiency gains will be wiped out in less than four years.

In an intriguing trend, some companies are dealing with apparently unquenchable growth in data traffic by beginning to move away from thousands of servers based on PC technology towards huge single computers with lower total energy costs. Who said the mainframe was dead?


The energy problem A generation ago, computer engineers would often speculate that software would eventually become more expensive than hardware. Their prediction turned out to be accurate. Today, similar people talk of the time when the hardware will cost less than the electricity to operate it. Perhaps that day has already arrived. A cheap server uses about £150 of electricity a year and you can buy one from Dell for £200. The lifetime electricity cost is far more than the purchase price.

The 25m or so servers in the world use an average of more than 200 W each. But running the machines is only about one third of the total power used in a data centre. The servers need to be cooled, and the incoming AC power needs to go through two separate conversion processes to DC and back. Many data centres are over-equipped because IT people are terrified of the top management fury when machines fail. Highly excessive levels of redundancy are often built in. Poor knowledge of how to configure air conditioning systems means that large amounts of cooling power are wasted. IT infrastructure managers are very rarely responsible for the electricity bill, so energy efficiency is bottom of the list of priorities.

Concerns over the power use in data centres first arose in the 1999 internet boom. The worry was not driven by cost or by climate change fears. It arose because in some parts of the US the power companies simply didn’t have enough spare electricity. This continues to be a problem in America and elsewhere and Google has even talked of building its own generating capacity for the half a million servers it owns.

But attention is now moving to concerns over the likely long-term rise in the price of electricity and its impact on the cost of running data centres. However, we shouldn’t over-estimate the financial incentive to improve. BT’s centres cost about £20m a year to power. Google’s might take £75m or so. It is not small change, but the economics of these businesses do not depend on cutting power use in data centres. The climate change argument is more impressive. At the average carbon content of UK electricity, BT’s farms put out about 100,000 tonnes of CO2 a year. (BT can claim that this is an unfair point since it buys electricity exclusively from renewable sources.)

Electricity use in BT In its 2007 sustainability report, BT says that the electricity consumption in its networks was 1,992 GWh - just over half a percent of the UK total. Unusually, the report does give comparable figures for last year. Instead BT says, ‘Our investment in energy management has helped us to keep our energy consumption relatively unchanged’. A look in the archives suggests that this is perhaps a slightly questionable summary: electricity use in the networks and data farms has risen by about two and a half percent, a somewhat higher figure than for the UK as a whole.

No one could be surprised at the growth. BT’s underlying business is growing at least as fast as this. Slightly more surprisingly, BT’s use in its offices rose by almost 3%. As in most businesses, increased personal computer power and an increasing number of office peripherals probably drove this figure upwards. But office use is little more than 10% of its total electricity demand. Focus will be on the networks, including the data centres.

According to the company, data centre electricity use is currently rising at about 40% a year as broadband gets into more and more UK homes, and businesses store and retrieve more data. Total electricity use last year was 230 GWh. Although this figure is only about 12% of BT’s electricity use outside its offices, the figure is rising so fast that it will wash away any energy use reductions the company makes elsewhere.

The tactics for energy use reduction in data centres In its new data centres BT is making some radical changes to the way it does things:

  • Wherever and whenever possible it is avoiding the use of air conditioning.
  • It is running the data centres entirely on DC power.
  • It is making use of what is called ‘virtualisation’, enabling one server to emulate two or more. This consolidation means that a smaller number of boxes are needed.

Avoiding air conditioning In a traditional data centre, keeping the temperature to 20 degrees or less uses more power than running the computers. An analysis from the Green Grid, a new industry information sharing group, suggests that only about one third of the power going into a data centre is actually used by computers.

The Green Grid figures are as follows:

Chilling 33%
De-humidification 3%
Computer room air conditioning apparatus 9%
The IT kit 30%
Power distribution 5%
Power supply and battery storage 18%
Switchgear and generator 1%
Lighting 1%

BT has decided that it can run its data centre at much higher temperatures than used to be considered conventional. At a presentation to analysts, it told us that the wisdom that IT equipment has to be run at 20 degrees is backed by little data. In fact, the company said that the standard was set by IBM a generation ago. Early IBM storage devices used magnetic tape. Apparently the tape stretched unacceptably at temperatures higher than 20 degrees and air conditioning was needed. The conventional wisdom remains, even though tape drives are no longer in use.

BT claims that its servers can now run at 40 degrees with no observable impact on failure rates. It has achieved this better performance partly by obliging its suppliers to design and manufacture their kit to work reliably at these higher temperatures. A top limit of 40 degrees enables BT to shift to fresh air as it main coolant in its new server farms.

Blowing coolish air over the servers from the outside will keep the boxes under 40 degrees for a very large fraction of the year. BT mentions an 85% reduction in air conditioning costs. It intends to go further and says it will be encouraging suppliers to design products that work well at 50 degrees.

Not using air conditioning has the side benefit of minimising the need for dehumidification. Air conditioning can produce condensation on equipment which needs to be wicked away by dehumidifiers.

Running on DC power Most data centres buy AC power, turn it into DC for battery storage (uninterruptible power supply) and then convert back to AC before the final switch to DC to power the processors. Each transition loses some energy, though the amount will depend on how modern the equipment is. In total, the losses can be almost as great as the cost of running the servers themselves.

Changing the electricity to low voltage DC inside the building means conversion losses are minimised. Moving from AC to DC or the reverse creates more heat that has to be evacuated from the building, so BT’s move to DC only in new data centres is helping reduce the demand for air conditioning.

‘Virtualisation’ Creating more than one virtual server inside one box is an effective means of decreasing energy use. It means that each physical box can typically be used to serve more data. In effect it improves capacity utilisation.

In total, BT believes that it can shave 60% of total electricity demand in a new data centre, reducing typical electricity use to 3.2 MW. (Such a centre would still take all the power from one of the largest wind turbines working at full speed.) The electricity cost will still be over £2m a year but BT claims a payback period of less than 18 months on the extra investment in temperature resilient hardware.

The Green Grid BT is a member of the rapidly growing Green Grid, a US-based organisation dedicated to spreading best practice in data centre energy use. Interestingly, Green Grid is initially focusing on far less radical steps to reduce power use:

  • More effective use of air conditioning rather more than completely avoiding it. US summer temperatures will generally be much higher than in the UK, but data centre operators still have scope for using fresh air cooling for a large portion of the year.
  • Ensuring that the equipment is the correct size and capacity for the job. In effect, it is saying that data centre managers are only ever measured on the percentage of time the centre is working properly (‘uptime’) or on response times to data requests. This is generally resulting in large-scale over-engineering of most data centres, at considerable cost in electricity and capital expenditure.
  • Green Grid also focuses on ensuring that servers can be used for multiple tasks (‘virtualisation’). The logical extension of this policy is the rebirth of the mainframe, one piece of physical hardware that can allocate parts of its capacity to different tasks, allocating processing capacity as required. If electricity gets really expensive, we may see machines in our data centres that look suspiciously like modern versions of the IBM mainframe computers of the 1970s.

Much of the global data in this article comes from the extremely impressive analysis of Jonathan Koomey at http://enterprise.amd.com/Downloads/svrpwrusecompletefinal.pdf.